Delimiters

When using the PCRE functions, it is required that the pattern is enclosed
by delimiters. A delimiter can be any non-alphanumeric,
non-backslash, non-whitespace character.

Often used delimiters are forward slashes (/), hash
signs (#) and tildes (~). The
following are all examples of valid delimited patterns.

/foo bar/
#^[^0-9]$#
+php+
%[a-zA-Z0-9_-]%

It is also possible to use
bracket style delimiters where the opening and closing brackets are the
starting and ending delimiter, respectively. (),
{}, [] and <>
are all valid bracket style delimiter pairs.

Bracket style delimiters do not need to be escaped when they are used as meta
characters within the pattern, but as with other delimiters they must be
escaped when they are used as literal characters.

If the delimiter needs to be matched inside the pattern it must be
escaped using a backslash. If the delimiter appears often inside the
pattern, it is a good idea to choose another delimiter in order to increase
readability.

/http:\/\//
#http://#

The preg_quote() function may be used to escape a string
for injection into a pattern and its optional second parameter may be used
to specify the delimiter to be escaped.

You may add pattern
modifiers after the ending delimiter. The following is an example
of case-insensitive matching:

User Contributed Notes 1 note

Note that bracket style opening and closing delimiters aren't a 100% problem-free solution, as they need to be escaped when they aren't in matching pairs within the expression. That mismatch can happen when they appear inside character classes [...], as most meta-characters lose their special meaning. Consider these examples: